The invention relates to a method and arrangement for cleaning the screening device of screen while separating fine material and coarse material from material conducted to the screen with the aid of the screen according to the preambles of the independent claims presented below.
The characteristics of screens often include that the screens clog in some way. The occurrence of the phenomenon depends of course largely on the consistency and characteristics of the material to be screened. However, it is fairly common that the screening device of the screen, generally the screen mesh or screen plate clogs and the operation of the screen is disturbed, or even ceases. The screening effect may in a short period of time decrease to a fraction of the normal and then measures have to be taken to restore operational capacity of the screen. Such work is often quite cumbersome, as the screen may be large and it may have to be not only emptied but also partly disassembled in order to replace some parts with new or repaired ones. This is normally very time-consuming, whereby the screen is totally out of service and even disruptions to the manufacturing process may occur. Arrangements are known, where the screen is attempted to be cleaned with the aid of pressurised air, but is has been observed that such solutions are relatively ineffective and the screen must in any case most often be opened and for the larger part disassembled and reassembled. The work is often dirty, even hazardous.
The object of the invention is to avoid the above-mentioned disadvantages and to present a screen solution, in which the screen does not have to be opened for cleaning at all and in which the cleaning can be done while the screen is in operation. A further object of the invention is to present a solution where either separated fine material or some external material suitable for the purpose is used for the cleaning of the screen or at the same time for adding of material to the screen and thereby simultaneously in the material that passes through the screening apparatus.
The object of the invention is achieved in the manner disclosed in the independent claims and other claims presented hereinafter. The method according to the invention is used for cleaning the screening device of the screen while the screen is in operation. At least said screening device is arranged in the screen, which screening device has openings for separating components of different sizes of the material to be screened into fine material and coarse material, an actuator means for moving the screening device, a conveying means for conducting the material to be screened to the screening device of the screen and a conveying means for moving the fine material that has passed through the screening device and means for conducting the separated coarse material away from the screen. If a material is blown for cleaning the screening device at least at intervals among the pressure medium in the opposite direction regarding the fine material passing through the screening device, the particle size of which material being at least mainly smaller than the size of the openings in the screening device, the screening device of the screen is efficiently cleaned of the fine material to be screened caught in the openings of the screening device and thus the screening device of the screen is cleaned at the spot where the blow is directed and the operational ability of the screen is at least near the optimal.
If gas is used as the pressure medium, in which at least the main part is pressurised air, the pressurised air has no detrimental effect to most materials to be screened. Pressurised air is also commonly available in such places where screens are used. Furthermore, air is inexpensive and not dangerous.
If material is added to the pressure medium which mainly comprises fine material that has passed through the screening device, the cleaning of the screening device is effective and the screen is easily maintained in operational and effective status. During operation of the screen said fine material is furthermore always available for such a circulation in the screen and therefore it is not necessary to arrange for the acquisition or adding of any additional material.
However, if material is added to said pressure medium which material is used for changing of characteristics of the material that passes through the screen, particularly fine material separated with the aid of the screening device, it is possible in this manner conveniently not only to add some suitable material to the screen but also to clean the screen during operation and to maintain it in working order and effective.
If the pressure medium and the material to be blown therewith is blown towards the screening device at least through one nozzle, an efficient effect is achieved by directing the blowing in such a manner.
If the blowing of the pressure medium and the material to be blown thereamong is arranged to be carried out at intervals the operating life span of the screening device of the screen can be lengthened, as the blow and the material travelling therewith wear the screen out.
If there is a plurality of said nozzles and if said blow is arranged only with one or at the most a few nozzles at a time, it is possible to clean almost the entire area of the screening device of the screen even with a relatively small amount of the pressure medium to be blown. No problems will arise regarding the removing of the pressure medium to be blown from inside of the screen because of the relatively small amounts.
If a control unit is to be used for guiding the operation of said blowing nozzles, the operation of which unit is guidable, the suitably effective cleaning effect can be chosen for each purpose of use and for each operating situation.
If the screening device and the blow nozzle are arranged to be in movement with regard to each other, the cleaning effect is efficient within a large area, even within the entire area of the openings of the screening device.